How can I combine multiple textures so that they blend together?
The traditional way to do this was vertex colors. The amount of (say) red would be the blend factor between the textures. Use a node setup like this.

Set the 3D View to Material preview and switch to Vertex Paint mode. Paint black where you want the first texture to show through, and red where you want the second.

Alternatively, you could use a third texture to control the blend amount and paint it in Texture Paint mode.